Materials which have high coercive forces (Hc), and smooth surfaces have been regarded as good media for high density recording. As recording media for this purpose, recording media based on magnetic powders and recording media based on thin metal films formed by the evaporation, metal plating or other processes have been used. However, both types of recording media suffer from a lack of stability because they are liable to be oxidized, and attempts to put them to practical use have been unsuccessful.
In recent years, to attain high output levels in short wavelength recording systems, recording systems which utilize (1) the vertical component of the magnetic field used for recording or (2) perpendicular magnetic recording have been proposed.
Although these systems are excellent in principle, excellent recording media useful therein are not available.
Specifically, using a magnetic material made of conventional needle-shaped magnetic particles oriented at random has been proposed. However, such a material does not provide sufficient output due to the low relative density of the component oriented in the vertical direction.
Further, the use of needle-shaped magnetic particles oriented perpendicular to the plane of the magnetic layer has been proposed, and a number of methods for orienting such magnetic particles have also been proposed. However, sufficient orientation has not been achieved with these methods as the initially vertically oriented needle-shaped particles tilt down toward a plane parallel to the surface of the magnetic layer upon contraction of the magnetic coating in its thickness direction on drying.
In addition, recording media comprising a thin metal film formed by sputtering or evaporation techniques for perpendicular magnetic recording have been proposed. However, such as insufficient in terms of chemical stability, magnetic head abrasion resistance and durability.
In the present invention, the term "perpendicular magnetic recording" refers to the recording process in which a magnetic head as illustrated in FIG. 1 is used; such a process is described in Television Gakkaishi, Vol. 32, No. 5, P399 (1978) or IEEE Transactions on Magnetics 1980 (1) Vol. MAG-16, November 1 P71.
On the other hand, the expression "the recording system utilizing the vertical component of magnetization" refers to the recording system in which the vertical component of a magnetic field created by a ring head on the surface of a magnetic layer, as illustrated in FIG. 2, is utilized; such a system is described in IEEE Transactions on Magnetics Vol. 15, No. 6, November 1979. When a head having a narrow gap is used, recording is, in general, carried out very efficiently.
With the invention of solving these problems, we developed a magnetic recording medium which contains plate-shaped magnetic particles whose major planes are oriented parallel to the surface of the magnetic layer, which they are in, that is to say, whose axes of easy magnetization are aligned perpendicular to the surface of magnetic layer which they are in. Such a recording medium provides a higher output level and a higher signal to noise ratio in the short wavelength recording as compared with conventional recording media. However, the values thereof were still not high enough.
As a result of examining the causes of such deficiencies it turned out that this is mainly due to the fact that when the wavelength of a wave to be recorded is smaller than size of the magnetic particles involved, magnetization induced in an individual magnetic particle entirely or partially counteracts the magnetization on another individual particle due to the single domain structure which the magnetic particles have, as illustrated in FIG. 3(B) hereinafter, and only the slight residual magnetization can contribute to the reproduction output.
The development of plate-shaped magnetic particles having smaller sizes may be one way to solve this problem. However, this is not advisable because of undesirable side effects, e.g., fine granulation results in a reduced dispersibility, a reduced orientation and, from the industrial view, a lowering of filtration efficiency in washing.